The exchange of information between a contactless object and a contactless transceiver system is generally accomplished by remote electromagnetic coupling between the first antenna located in the contactless object and the second antenna located in the contactless transceiver system. Furthermore, the object is equipped with an electronic module featuring the first antenna connected to an electronic chip which contains, among other elements, a radio-frequency (RF) part, a microprocessor and/or a memory in which the information to be provided to the contactless transceiver system and the logic functions required to compile the information to be transmitted and to process the information received.
The contactless object, which may be a ticket or credit card format card, is a system which is being increasingly used in various sectors. For example, in the transportation sector, the disposable contactless ticket and the contactless smart card were developed as a means of payment for both occasional and regular users. The same holds true for the electronic wallet. Many companies have also developed identification means for their personnel using contactless smart cards.
At present, data transmissions between the contactless transceiver system, commonly referred to as the reader, and contactless smart cards are subject to ISO standards. Among the most widely spread, the standard ISO 14443 concerns data transmission via radio between a smart card and a reader and vice versa. This standard covers two transmission protocols known as type “A” transmission protocol and type “B” transmission protocol. These two data contactless data transmission protocols, A and B, differ in terms of the type of modulation used for Radio Frequency (RF) communication between the reader and the card on the one hand, and the card and the reader on the other hand. Only the signals transmitted from the reader to the card will be dealt with here.
In the direction of data transmission from the reader to the card, protocol B provides amplitude modulation corresponding to a modulation rate of 10 percent of the signal emitted or of the electromagnetic carrier wave by the data transmitted, while in protocol A, the electromagnetic carrier wave is modulated at 100 percent of its amplitude by the data transmitted. In both cases, the amplitude of the periodic electromagnetic carrier wave emitted is maximum by default during the first time interval t1 then, during the second time interval, for the first case, it is equal to approximately 82 percent of the maximum amplitude during the modulation while in the second case, it is equal to 0 percent of the maximum amplitude during the modulation time.
At present, an increasing number of standards require that contactless readers to be compatible with both transmission protocol types, A and B. The standardized electromagnetic carrier wave frequency is common to both protocols and is generally equal to 13.56 MHz.
One of the major performance criteria for a reader is the range of the electromagnetic radiated field which must be as large as possible. Thus, manufacturers attempt to develop the range of their transmission system by means other than increasing the voltage source. However, the increase in range must not present the risk of saturating or destroying the card when placed near the reader.
One of the factors for satisfying this performance criterion is the use of antennas which have a high overvoltage ratio. At the resonance frequency, the rms voltage at the terminals of the inductance, a source of electromagnetic carrier waves, is substantially equal to Q times the voltage at the terminals of the circuit; Q being the overvoltage factor. In this manner, the more the antenna presents a high overvoltage ratio, the greater is the range of its radiated field.
In order to obtain modulation of the signal emitted at 100 percent of its amplitude, the method commonly used at present consists in switching the voltage source off at the terminals of the circuit for the time corresponding to the split of the field according to protocol A, in order to stop the electromagnetic carrier wave from being transmitted.
In practice, switching off the generator drops the voltage to zero although significantly increases the impedance of the antenna's driving circuit. This results in the antenna continuing to emit owing to the loads accumulated in the circuit, which results in a dampened and oscillating emitted electromagnetic carrier wave for a time greater than the splitting time. As a result, during the signal split, the amplitude of the field radiated toward the card is not zero and the field emitted by the antenna thus does not correspond to a modulation of 100 percent of the amplitude of the electromagnetic carrier wave; this takes place for a time less than the theoretical split time, the amplitude being less than or equal to 5 percent of the maximum amplitude of the radiated signal.
Consequently, the use of an antenna with a high overvoltage ratio is compatible with a B type reader. For such a reader, the electromagnetic carrier wave is always active as it is modulated at only 10 percent of its amplitude while it is poorly compatible with an A type reader. Furthermore, the small dampening effect obtained with an antenna having a high overvoltage ratio only slightly influences the shape of a wave modulated at 10 percent of its amplitude.
In order to use an antenna which is compatible for both reader types, a first solution consists in using an antenna having a reduced overvoltage ratio and with damping compatible with the requirements of standard A, although at the expense of the performance characteristics under standard B.
A second solution consists in using a linear amplifier with bipolar transistors, which enables the signal emitted by the antenna to be dampened so that the radiated field is zero during the splitting time. This solution allows the expected damping to be obtained at the expense of yield. The circuit has constant output impedance, although requires high polarization currents. Implementation of the circuit is more complex.
A third solution consists in using field effect transistors for switching, in order to reduce losses and to not penalize the range of the B type reader. Using such transistors switches the circuit between the “open” and “closed” positions and allows a maximum radiated field to be obtained by increasing the impedance presented when the circuit is open sustainably but at the expense of the wave form in the case of the type A protocol.